1. Field of the Invention
The present invention relates to a technology for suppressing corrosion of metal components, such as nuclear power plant's structural components, in contact with high-temperature water, and more particularly, relates to a nuclear power plant having a corrosion-resistant coating, a method of making such a corrosion-resistant coating and a method of operating the nuclear power plant.
2. Related Art
Metal components exposed to high temperature environment are found in almost all of the modern industrial and commercial plants. For example, in the course of steam reforming in a hydrogen production chemical plant, the reaction is carried out at high temperatures and pressures. Inside a boiler or a metal pipe connected to the boiler, hot water and steam move or travel while causing corrosion. The conventional preventive measures against corrosion of metal components have involved use of expensive, special corrosion-resistant materials, improvements on the environment to which the metal components are exposed, etc. For example, in a thermal power plant, a pH control reagent, a deoxidizer, or the like is added to control water chemistry and to thereby reduce the corrosion.
In a boiling-water nuclear power plant, oxygen, hydrogen peroxide, and the like produced by radiolysis of water in the radiation field exist in a state dissolved in the reactor water. It is a well-known fact that stainless steel and nickel-based alloys, which are used for reactor structural components of the nuclear power plant, generate stress corrosion cracking in the presence of oxygen and hydrogen peroxide in a high-temperature environment such as a nuclear reactor.
Hydrogen injection of injecting hydrogen into the reactor water has applied to some BWR plants in the world to reduce oxygen and hydrogen peroxide dissolved in the reactor water (refer to GENSHIRO MIZU-KAGAKU HANDBOOK [Handbook of Water Chemistry of Nuclear Reactor System], edited by Atomic Energy Society of Japan, published by Corona Publishing Co., Ltd., on Dec. 27, 2000, p. 210). The effect of the oxygen and hydrogen peroxide reduction by the hydrogen injection is confirmed as the decrease in corrosion potential of the metal components. The generation of stress corrosion cracking and the crack growth rate depends on the corrosion potential. The lower the corrosion potential, more suppressed the generation of stress corrosion cracking and development of cracks. As a result, the lifetime of the metal components can be extended.
Other nuclear power plants in and outside Japan employ noble metal injection technology of conducting hydrogen injection after deposition of a noble metal, such as platinum (Pt) or rhodium (Rh), on surfaces of reactor structural components to accelerate reaction with hydrogen, increase the anode current to thereby decrease the corrosion potential (see the specification of Japanese Patent No. 2624906).
The meaning of the corrosion potential of the metal components is as follows. When a metal is immersed in an electrolyte, the metal shows a potential inherent to that metal. This potential is called “spontaneous potential” of that metal. A corroded metal material shows a potential different (polarized) from its spontaneous potential due to the corrosion reaction. This difference in potential is referred to as the “corrosion potential”. A continuous measurement of the potential difference will estimate the progression of the corrosion.
In a uniformly corroded metal material, the cathode reaction (reduction reaction) and the anode reaction (oxidization reaction) reach an equilibrium at the intersection of the cathode reaction polarization curve and the anode reaction polarization curve. This intersection corresponds to the corrosion potential.
Another approach that has recently drawn much attention for decreasing the corrosion potential is to utilize the photocatalytic reaction. By coating surfaces of the metal components with a photocatalyst and irradiating the photocatalyst with light having wavelength near ultraviolet, electrons activated by the photoexcitation reaction cause the corrosion potential to decrease. The photoexcitation reaction can be accelerated with a noble metal disposed nearby.
Accordingly, by depositing a photocatalyst or a high-efficiency photocatalyst containing a noble metal onto surfaces of the reactor structural components and inducing photoexcitation reaction by Cerenkov radiation generated in the reactor core, the corrosion potential during operation can be reduced (for example, refer to Japanese Patent Laid-open Publication Nos. 2001-4789 and 2001-276628).
As the method of preventing parts of the metal components from corrosion in the absence of light, a technology for decreasing the potential difference by generating thermostimulated current utilizing thermal energy instead of light energy has been suggested (refer to Japanese Patent Laid-open Publication No. 2003-232886).
Another corrosion reduction method proposed is to alternately laminate N-type semiconductor coatings and P-type semiconductor coatings onto surfaces of metal components (refer to Japanese Patent Laid-open Publication No. 9-125283). Yet another corrosion method proposed is to provide a coating consisting of three or more alternately stacked layers of an anion-permselective substance and a cation-permselective substance (refer to Japanese Patent Laid-open Publication No. 11-12719).
According to the technology disclosed in Japanese Unexamined Patent Application Publication No. 2001-4789, electrons irradiated with light are activated by the photoexcitation reaction, thereby generating electrical current that decreases the corrosion potential. The corrosion prevention effect is, however, rarely expected in parts not exposed to light.
In contrast to the corrosion prevention technology utilizing photoexcitation, a technology of decreasing the corrosion potential by utilizing electrical current produced by thermostimulated electrons is disclosed in Japanese Patent Laid-open Publication No. 2003-232886. According to this technology of producing the thermostimulated current, holes generated by thermostimulation cause anode reaction to occur and thereby increase the current. In an actual cases, however, the electrons stimulated by heat recombine with holes generated by the same thermostimulation, and the electric current does not easily flow. In order to efficiently convert the stimulated electrons and holes into a flow of electrical current, charge separation needs to be reliably carried out. Furthermore, the state that allows charge separation needs to be constantly maintained, and the ambient environmental conditions to which the metal components are exposed must be taken into consideration.
In addition, nuclear power plants have inside a substantially large number of narrowed parts and parts with complicated shapes. Thus, the corrosion prevention methods utilizing the semiconductor properties disclosed in Japanese Patent Laid-open Publication Nos. 9-125283 and 11-12719 would face difficulty in application.